The race to become the next mayor of London gets under way officially today with Boris Johnson ahead in the polls and Ken Livingstone due to warn voters that the capital faces a stark choice that will shape London for decades.

The latest opinion poll suggests Johnson, the Conservative candidate, has a 12-point lead over Livingstone, and last night bookmakers said the Henley MP was now the clear favourite for the May 1 poll.

Livingstone will launch his campaign for a third term at the Royal Festival Hall in London where he will tell supporters to prepare for the hardest and most bitter contest since he came to power eight years ago.

The Labour candidate will say: "Some people attempt to portray this election as about personality - a celebrity big brother in which the biggest issue that needs to be discussed is less than 400 bendy buses, a fraction of the bus service. It is not. The stakes are very high. It is about the most vital issues for the future of London affecting the lives of all Londoners for years."

He will highlight upcoming decisions on transport, housing, crime and the environment as proof of the candidates' different visions for London's future. "I believe Londoners do not want to turn the clock back - by a majority they want to continue to move forwards together."

Johnson, who last week said a victory in the mayoral race would provide the Conservative party with a "fantastic platform" for the next general election, said the mayor had "lost interest in Londoners".

In a video broadcast on his campaign website last night, Johnson said: "Ken Livingstone seems more interested in looking after his friends and cronies, fighting his own political causes and playing divisive political games ... his passion for London has withered as his obsession with power for its own sake has grown."

Yesterday's YouGov poll put Johnson on 49%, Livingstone on 37% and Liberal Democrat candidate Brian Paddick on 12%. The survey for the Evening Standard revealed 17% of those polled had not decided who they would vote for.

The Livingstone camp played down the findings saying the mayor always did badly in YouGov surveys, adding that the latest internal Labour party polling had put the mayor narrowly in front. However, the strength of Johnson's challenge was underlined by bookmakers Ladbrokes who said Johnson was now 4-6 favourite for May 1 with Livingstone at 5-4.

The swing to Johnson has coincided with repeated allegations of corruption at City Hall that resulted in the resignation earlier this month of Livingstone's race adviser Lee Jasper.

The Livingstone camp, which says it will win if the campaign focuses on policies, was boosted yesterday when former BBC chairman Greg Dyke voiced his support.